This invention concerns a wafer structure for forming a semiconductor single crystal film in which a single crystal film of a semiconductor is formed on an insulation layer.
Operation speed and density of semiconductor integrated circuits have been increased in recent years susbtantially to their limit obtainable by conventional inter-device separation technics. In view of the above, an attention has now been attracted to SOI (Silicon On Insulator) structures or SOI technics that can improve the reliability upon operation by completely separating the insulation layer as far as the bottom face of a device in addition to the separation only for the lateral direction, as well as can attain an increased speed due to the reduction in the parasitic capacitance.
FIG. 4 of the appended drawings is a perspective view showing a typical device for use in a zone melting method capable of collectively obtaining single crystal films on a large area wafer in a short time, in which are shown a lower heater 11, a linear heater 12, a specimen wafer 13 and a melting zone 14 in the specimen wafer 13.
Referring to the melting method, the specimen wafer 13 is heated at about 1200.degree. C. by the lower heater 11, as well as a local temperature increased region is formed by the linear heater 12 from above to form a stripe-like melting zone 14 on the surface of the specimen wafer 13 and a polycrystalline semiconductor layer deposited on the specimen wafer 13 is converted into single crystals by moving the linear heater 12 in the direction of the arrow 10.
FIG. 5 shows the structure of the specimen wafer employed so far in the zone melting method described above. The specimen wafer 13 comprises a thick silicon dioxide layer 16 of about 0.5 .mu.m formed on one main surface of a single crystal silicon substrate 15 and a polycrystalline silicon layer 17 to be melted and recrystallized is formed to a thickness of about 0.5 .mu.m on the surface of the silicon dioxide layer. Further, a silicon dioxide layer 18 of about 2 .mu.m and a silicon nitride layer 19 of about 30 .mu.m are formed in lamination as the protection layer on the surface of the polycrystalline silicon layer 17 for preventing the layer 17 from peeling off the silicon substrate 15 upon melting.
Since the conventional wafer structure for forming the semiconductor single crystal film has been constituted as described above, the polycrystalline silicon layer 17 to be used as the semiconductor device after the recrystallization is sandwiched between relatively thick insulation layers, in which no satisfactory heat dissipation can be obtained. Particularly, since the region of the polycrystalline silicon layer 17 heated to an extremely high temperature upon melting and recrystallization has a width as large as about 2 mm relative to its thickness of about 500 A, it is extremely difficult for dissipating latent heat upon solidification and for controlling the temperature distribution and the setting temperature required for preventing boiling in the melting zone 14. Although the melting zone 14 may be considered linear in macro point of view, the solid-liquid interface is never linear when examined finely. This is caused due to the difference of the growing speed on every directions of the crystal face and, accordingly, the face (111) having the lowest growing speed constitutes the solid-liquid interface as shown in FIGS. 6a and 6b (the silicon dioxide layer 18 and the silicon nitride layer 19 are not illustrated in FIGS. 6a and 6b). Accordingly, the growing direction is not aligned with but somewhat inclined to the scanning direction of the heating source. Thus, distortions are induced in the illustrated regions 30 where the growing faces meet with each other and, finally, small-angled grain boundaries are generated to loss the conditions for the growth of the single crystals. The sites in which such small-angled grain boundries are generated can not be controlled at all by the conventional wafer structure and, accordingly, this causes degradation for the uniform characteristics of the device formed to the crystalline layer.